The thiosulfate ion, S2O32−, is a structural analogue of the SO42− ion in which one oxygen atom is replaced by one S atom. However, the two sulfur atoms in S2O3−2 are not equivalent. One of the S atoms is a sulfide-like sulfur atom that gives the thiosulfate its reducing properties and complexing abilities.

Thiosulfates are used in leather tanning, paper and textile manufacturing, flue-gas desulfurization, cement additives, dechlorination, ozone and hydrogen peroxide quenching, coating stabilizers, as an agricultural fertilizer, as a leaching agent in mining, and so on.
Due to these complex-forming abilities with metals, thiosulfate compounds have been used in commercial applications such as photography, waste treatment and water treatment applications.
Thiosulfates readily oxidize to dithionates, trithionates, tetrathionates, and finally to sulfates:2S2O32−+3O2→2S2O62−S2O62−+O2→2SO42−7S2O32−+3/2O2→2S3O62−+2S4O62−2S3O62−+6O2→6SO42−S4O62−+5O2→4SO42−
Due to this transformation, thiosulfates are used as fertilizers in combination with cations such as ammonium, potassium, magnesium and calcium. The ammonium, alkali metal and alkaline earth thiosulfates are soluble in water. Water solubilities of thiosulfates decrease from ammonium to alkali metals to alkaline earth thiosulfates.
Calcium is an essential plant nutrient. Calcium availability is essential in the biochemistry of plants, and as it has been learned recently, in the nitrogen fertilizer efficiency of surface-applied urea. The need of soluble calcium by high-value crops is different than the role of important soil amendments such as lime or gypsum. Both soluble calcium and these soil amendments are extremely important in soil fertility and plant nutrition and complement each other.
In the mining industry, thiosulfate leaching of gold is preferred over the conventional cyanide leaching due to the hazardous nature of cyanide. Calcium thiosulfate is an alternative replacement for lime/cyanide slurry in leaching of gold.
“Lime-sulfur” is a commonly used term for a calcium thiosulfate and calcium polysulfide mixture resulting from reaction of lime and sulfur. U.S. Pat. No. 1,685,895 describes the formation of a lime-sulfur solution from lump lime, lump sulfur, and hot water.
J. W. Swaine, Jr. et al. in U.S. Pat. No. 4,105,754 describe the production of calcium thiosulfate by a metathesis reaction of ammonium thiosulfate and calcium hydroxide or calcium oxide. This approach requires constant removal of ammonia by air stripping at below the boiling point of the mixture and capturing the gas.
Japanese Patent No. 6,039 issued in 1973 describes preparation of calcium and magnesium thiosulfate by treating sulfur and the corresponding sulfite in an alkaline solution. High yields are only obtained with magnesium thiosulfate. This patent also describes the formation of calcium thiosulfate from a salt exchange process between magnesium thiosulfate and calcium hydroxide.
Sodium thiosulfate and calcium chloride were used to produce calcium thiosulfate in Spanish Patent No. 245,171. The byproduct of this approach is a large amount of sodium chloride in the calcium thiosulfate product.
Lee, et al. in U.S. Pat. No. 4,976,937 describe the formation of a mixture of calcium polysulfide/calcium thiosulfate from a lime-sulfur mixture at 6-100° C. to be used for removal of sulfur dioxide from flue gases.
Vonkennel and Kimmig in U.S. Pat. No. 2,198,642 describe the production of stable calcium thiosulfate solution from calcium chloride and sodium thiosulfate.
Russian Patent No. RU 2167101 C2 describes the preparation of sodium thiosulfate and calcium thiosulfate from sulfur and a solution of sodium alkali or calcium alkali in stoichiometric amounts under autoclave conditions with an oxidizer.
Hojjatie, et al. in U.S. Pat. No. 6,984,368 B2 describe the preparation of calcium thiosulfate liquid fertilizer solution from lime, sulfur and oxygen. The patent describes the preparation of calcium thiosulfate in batchwise preparation.
The thiosulfate leaching of gold has been proven technically viable. For example, see U.S. Pat. No. 4,070,182, U.S. Pat. No. 4,269,662, and U.S. Pat. No. 4,369,061, which describe the use of ammonium thiosulfate in gold leaching. Application of copper-ammonium thiosulfate in the gold leaching process is described in U.S. Pat. No. 4,654,078. Choi, et al. in U.S. Pat. No. 7,572,317 describe the use of ammonium, sodium, and calcium thiosulfate in the leaching of gold.